LTM8056 datasheet(16/26 Pages) LINER

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LTM8073

8073fa

For more information www.linear.com/LTM8073

applicaTions inForMaTion

pulls up the TR/SS pin to about 2.4V. Putting an external

capacitor on TR/SS enables soft starting the output to

reduce current surges on the input supply. During the soft-

start ramp the output voltage will proportionally track the

TR/SS pin voltage. For output tracking applications, TR/SS

can be externally driven by another voltage source. From

0V to 0.8V, the TR/SS voltage will override the internal

0.8V reference input to the error amplifier, thus regulating

the FB pin voltage to that of the TR/SS pin. When TR/SS is

above 0.8V, tracking is disabled and the feedback voltage

will regulate to the internal reference voltage. The TR/SS

pin may be left floating if the function is not needed.

An active pull-down circuit is connected to the TR/SS pin

which will discharge the external soft-start capacitor in

the case of fault conditions and restart the ramp when the

faults are cleared. Fault conditions that clear the soft-start

capacitor are the RUN pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Prebiased Output

As discussed in the Output Voltage Tracking and Soft-Start

section, the LTM8073 regulates the output to the FB volt-

age determined by the TR/SS pin whenever TR/SS is less

than 0.8V. If the LTM8073 output is higher than the target

output voltage, the LTM8073 will attempt to regulate the

output to the target voltage by returning a small amount

of energy back to the input supply. If there is nothing load-

ing the input supply, its voltage may rise. Take care that

it does not rise so high that the input voltage exceeds the

absolute maximum rating of the LTM8073.

Frequency Foldback

The LTM8073 is equipped with frequency foldback which

acts to reduce the thermal and energy stress on the inter-

nal power elements during a short-circuit or output over-

load condition. If the LTM8073 detects that the output

has fallen out of regulation, the switching frequency is

reduced as a function of how far the output is below the

target voltage. This in turn limits the amount of energy

that can be delivered to the load under fault. During the

start-up time, frequency foldback is also active to limit

the energy delivered to the potentially large output capaci-

tance of the load. When a clock is applied to the SYNC

pin, the SYNC pin is floated or held high, the frequency

foldback is disabled and the switching frequency will slow

down only during overcurrent conditions.

Synchronization

To select low ripple Burst Mode operation, tie the SYNC

pin below about 0.4V (this can be ground or a logic low

output). To synchronize the LTM8073 oscillator to an

external frequency connect a square wave (with about

20% to 80% duty cycle) to the SYNC pin. The square

wave amplitude should have valleys that are below 0.4V

and peaks above 1.5V.

The LTM8073 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

instead will pulse skip to maintain regulation. The LTM8073

may be synchronized over a 200kHz to 3MHz range. The RT

resistor should be chosen to set the LTM8073 switching

frequency equal to or below the lowest synchronization

input. For example, if the synchronization signal will be

500kHz and higher, the RT should be selected for 500kHz.

For some applications it is desirable for the LTM8073 to

operate in pulse-skipping mode, offering two major dif-

ferences from Burst Mode operation. The first is that the

clock stays awake at all times and all switching cycles

are aligned to the clock. Second is that full switching fre-

quency is reached at lower output load than in Burst Mode

operation. These two differences come at the expense

of increased quiescent current. To enable pulse-skipping

mode, the SYNC pin is floated.

The LTM8073 features spread spectrum operation to fur-

ther reduce EMI/EMC emissions. To enable spread spec-

trum operation, apply between 2.9V and 4.2V to the SYNC

pin. In this mode, triangular frequency modulation is used

to vary the switching frequency between the value pro-

grammed by RT to about 20% higher than that value. The

modulation frequency is about 3kHz. For example, when

the LTM8073 is programmed to 2MHz, the frequency will

vary from 2MHz to 2.4MHz at a 3kHz rate. When spread

spectrum operation is selected, Burst Mode operation is

disabled, and the part will run in pulse-skipping mode.

The LTM8073 does not operate in forced continuous

mode regardless of SYNC signal.

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